A well known problem with untreated urea is the loss of nitrogen as ammonia through a process called volatilisation, which leads to lower crop responses. In addition, this lost ammonia also has an environmental impact, causing acidification.

The difference in crop performance between urea and ammonium nitrate had long been assumed to be similar, but trials five to 10 years ago indicated ammonia losses from urea were quite large, or around 24%.

However, the review published by ADAS in the journal of Agricultural Science shows that the loss in crop performance is smaller than the loss of ammonia, with an average 7% lower performance for untreated urea compared with ammonium nitrate.

“Growers, therefore, need to apply a bit more untreated urea to get the same agronomic response,” he says.

Prof Sylvester-Bradley advises growers assume a 10% difference, thereby applying 10% more on a nitrogen basis to gain the same crop responses.

Looking at treated urea, there are fewer trials, but the data show treated urea is as good as ammonium nitrate for agronomic performance.

One example is Koch Advanced Nitrogen (KaN), which consists of urea and a urease inhibitor, developed by Agrotain International, explains Simon Borthwick of Koch.

“We were pleased to read that ADAS’s confirmation of the high yields with the treated urea. The nitrogen stabilizer maximises the nitrogen retained after application to support optimal crop growth.”

One advantage with reducing ammonia losses is that the greenhouse intensity is reduced. Prof Sylvester-Bradley suggests that it gives growers another option, being as good as ammonium nitrate in agronomic performance while reducing their greenhouse gas emissions compared to straight urea or average ammonium nitrate.

The review also shows that ammonium nitrate manufacturers that now use best available technology have massively reduced greenhouse gas footprints of their products compared with average ammonium nitrate.

Speaking to Farmers Weekly, GrowHow UK highlighted that it had invested heavily in production efficiency, a major recent project being the installation of N2O abatement technology at its Billingham site. “This improvement alone has reduced the carbon footprint for Nitram by 40%.”